1. Field of the Invention
The present invention relates to pyrrolo-pyrazole derivatives, to a process for their preparation, to pharmaceutical compositions comprising them, and to their use as therapeutic agents, particularly in the treatment of cancer and cell proliferation disorders.
2. Discussion of the Background
Several cytotoxic drugs such as, e.g., fluorouracil (5-FU), doxorubicin and camptothecins, damage DNA or affect cellular metabolic pathways and thus cause, in many cases, an indirect block of the cell cycle. Therefore, by producing an irreversible damage to both normal and tumor cells, these agents result in a significant toxicity and side-effects.
In this respect, compounds capable of functioning as highly specific antitumor agents by selectively leading to tumor cell arrest and apoptosis, with comparable efficacy but reduced toxicity than the currently available drugs, are desirable.
It is well known that progression through the cell cycle is governed by a series of checkpoint controls, otherwise referred to as restriction points, which are regulated by a family of enzymes known as the Cyclin-dependent kinases (Cdk). In turn, the Cdks themselves are regulated at many levels such as, for instance, binding to cyclins.
The coordinated activation and inactivation of different Cdk/Cyclin complexes is necessary for normal progression through the cell cycle. Both the critical G1-S and G2-M transitions are controlled by the activation of different Cdk/Cyclin activities. In G1, both Cdk4/Cyclin D and Cdk2/Cyclin E are thought to mediate the onset of S-phase. Progression through S-phase requires the activity of Cdk2/Cyclin A whereas the activation of Cdc2/Cyclin A (Cdk1) and Cdc2/cyclin B are required for the onset of mitosis. For a general reference to cyclins and cyclin-pendent kinases see, for instance, Kevin R. Webster et al, in Exp. Opin. Invest. Drugs, 1998, Vol. 7(6), 865-887.
Checkpoint controls are defective in tumor cells due, in part to disregulation of cdk activity. For example, altered expression of cyclin E and cdks has been observed in tumor cells, and deletion of the cdk inhibitor p27 KIP gene in mice has been shown to result in a higher incidence of cancer.
Increasing evidence supports the idea that the cdks are rate-limiting enzymes in cell cycle progression and, as such, represent molecular targets for therapeutic intervention. In particular, the direct inhibition of cdk/cyclin kinase activity should be helpful in restricting the unregulated proliferation of a tumor cell.